Ascorbate (Asc) has long been known to be an antioxidant against reactive oxygen species (ROS) for cytotoxicity and human diseases. However, its reported reactivity with superoxide radical (O2•−), an important ROS involved in aerobic metabolism system, exhibits great discrepancy; its mechanistic mode of actions on the molecular level is also largely debatable. Here, we investigated the reaction of Asc and O2•− by a combination of experimental and theoretical approaches. The rate constant measured by a competitive technique method was determined to be 3.3 × 104 M−1 s−1 under physiological condition. Mechanistically, our theoretical result indicated that O2•− abstracts a hydrogen atom on the enolic hydroxyl group forming ascorbyl radical (Asc•), which is unreactive with O2•− or Asc• itself. However, with different protonation state of Asc• (i.e., AscH•) in acidic environment, its reactivity was demonstrated to be significantly altered. AscH• can react with O2•−via hydrogen atom abstraction and single electron transfer and/or undergo self−disproportionation. These complementary results provided fundamental principles for the ecotoxicological function and environmental potentials of Asc against ROS.

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